1. Field of the Invention
The present invention relates generally to shut-off valves, and, more particularly, it relates to a shut-off valve which automatically blocks flow from a gas cylinder when the pressure in the cylinder falls below a predetermined level.
2. Description of the Prior Art
When the pressure in a gas cylinder is depleted, the cylinder must be refilled. It is desirable to leave a small residual pressure in the cylinder to prevent its contamination before it is refilled by the gas supplier. Gas cylinders are typically provided with a manual shut-off valve at their outlet, and when the pressure in the gas cylinder falls below the minimum acceptable level, the user could close the valve to prevent the residual pressure from being lost.
Normally, however, the manual shut-off valve on such cylinders is left open by the user and the residual gas pressure is lost. The cylinder must then be cleaned and purged prior to refilling in order to meet the purity specifications for the particular gas. The cleaning of the cylinder is an extra step which increases the cost of the refilling.
It is thus desirable to provide an automatic shut-off means which will seal the outlet of the cylinder whenever the pressure therein falls below a predetermined level. Heretofore, this has been accomplished by providing a check valve in addition to the manual valve. The check valve is typically located in the outlet port of the manual shut-off valve. Such check valves are disclosed in U.S. Pat. No. 3,384,133 to Gordon and U.S. Pat. No. 4,210,168 to Yonezawa.
The use of a check valve separate from the mechanism of the manual shut-off valve is undesirable for two reasons. First, it increases the complexity and cost of the valve. Second, shut-off valves for gas cylinders must be made small enough to fit beneath a protective cover which is standard to the gas cylinder industry. The provision of a second valve mechanism within the shut-off valve body makes it more difficult to produce the compact valve required.
The inclusion of a separate check valve, however, has been necessitated by the broad range of pressures over which the cylinder operates. The manual valve must be capable of sealing against cylinder pressures in excess of 3,000 PSI while the automatic shut-off of the cylinder occurs with an internal pressure in the range from approximately 10 to 50 PSI. The combination of valve plug and seat required to shut off against very high pressures is unsuitable for automatic actuation by a spring set to shut-off against low pressures. That is, a valve plug having a hard seating surface, as required by high pressure operation, will not seal properly with a low seating force such as that provided by a spring set to seat against 15 PSI. On the other hand, the soft seat required for low pressure operation will deform and fail when subjected to high pressure. The problem is exacerbated because the check valve on the cylinder must allow no essential leakage since the cylinders will typically be shipped and stored for long periods prior to refilling.
Thus, while it would be desirable to provide a single valve plug and seat for both the manual and the automatic shut-off of a high pressure gas cylinder, such a combination is not found in the prior art.
Valves having a single valve plug and seat and dual actuating means are found in other applications, however. U.S. Pat. No. 3,211,175 to Replogle discloses a combination regulating the check valve. An automatic actuator may be adjusted to provide the desired outlet pressure from the gas cylinder. An integral check valve is provided to allow the gas cylinder to be filled without having to remove the gas regulator from the cylinder. The valve disclosed is provided in addition to a manual shut-off valve and therefore need not shut off against a wide range of pressures.
A combination metering, check and shut-off valve is disclosed in U.S. Pat. No. 3,420,493 to Kraft. The valve described therein is a conventional needle valve having a spring-loaded valve plug. A threshold flow of fluid through the valve holds the plug fully open in relation to an adjustable valve stem. Flow below this threshold level allows the valve plug to spring closed regardless of the position of the valve stem. Again, this valve would be incapable of sealing against a wide range of pressures.
Thus, while it is known to combine more than one function in a single valve plug and seat combination, they have not been adapted for for both the manual and automatic shut-off of a high pressure gas cylinder.